Angular 2: A property set during OnInit is undefin

问题:

I have this component:

export class CategoryDetailComponent implements OnInit{
  category: Category;
  categoryProducts: Product[];
  errorMessage: string;
  constructor(private _categoryService: CategoryService, private _productService: ProductService, private _routeParams: RouteParams ) {}

  ngOnInit() {
    this.getCategoryAndProducts();
  }
  getCategoryAndProducts() {
    let categoryName = this._routeParams.get('name');
    let categoryId = this.routeParams.get('id');
    var params = new URLSearchParams();
    params.set('category', categoryName);

    Observable.forkJoin(
      this._categoryService.getCategory(categoryId),
      this._productService.searchProducts(params)
    ).subscribe(
      data => {
      //this displays the expected category's name.
      console.log("category's name: "+ data[0].attributes.name)
      this.category = data[0];
      this.categoryProducts = data[1];
      }, error => this.errorMessage = <any>error
    )
  }
}

In the component's template I have this:

<h1>{{category.attributes.name}}</h1>

When I navigate to this component, I get an error:

TypeError: cannot read property 'attributes' of undefined

why is the category property on the template undefined and how can I solve this?

回答1:

The bindings in the template are evaluated before ngOnInit(). To prevent Angular to throw an error you can use

<h1>{{category?.attributes.name}}</h1>

The Elvis operator prevents Angular evaluating .attributes... unless category has a value.

回答2:

You can also solve this by initializing your variables.

Init this while declaring:

export class CategoryDetailComponent implements OnInit{
  category: Category = new Category();
  ...
  ...
}

OR init in Component constructor:

constructor(....) {
   this.category = new Category();
}